Automatic Repeat reQuest (ARQ) is a method for packet data transmission that uses positive or negative acknowledgement by the receiving party to indicate to the sending party whether the data packet has been successfully received or not. If the sender does not receive an acknowledgment or receives negative acknowledgment, it usually retransmits until an acknowledgment is received from the receiving party or the number of re-transmissions exceeds a predefined threshold. The term “Hybrid ARQ” (HARQ) is used to describe any scheme that combines forward error correction (FEC) with ARQ in which data received in unsuccessful attempts are used by receiving party in FEC decoding instead of being discarded. The simplest form of HARQ is called Chase Combining (CC) wherein each retransmission repeats the first transmission, or part of it, and the receiver combines multiple received copies of the coded packet prior to decoding. Incremental redundancy (IR) is another HARQ technique wherein instead of sending simple repeats of the entire coded packet, additional redundant information is incrementally transmitted if the decoding fails on the first attempt. HARQ is used in a number of wireless technologies such as 3GPP High Speed Packet Access (HSPA) and Long Term Evolution (LTE), 3GPP2 High Rate Packet Data (HRPD).
Successful reception may depend on the modulation and coding scheme (MCS) used to send the data packet. A lower order modulation scheme, meaning less number of bits transmitted per modulated symbol, typically provides better performance than higher order modulation in a given radio channel condition, but yields lower data throughput. For example, QPSK (quadrature phase shift keying) is more robust and can tolerate higher levels of communication errors than 16QAM (quadrature amplitude modulation). However, 16QAM provides a higher data rate than QPSK.
Link adaptation is a term used in wireless communications to indicate dynamic matching of the modulation and coding scheme to the radio channel conditions. For example, in case of 3GPP high speed downlink packet access (HSDPA), rate control is implemented by the medium access control high speed (MAC-hs) entity that configures the transport format of a high speed downlink shared channel (HSDSCH) in every 2 milliseconds of transmit time interval (TTI). This results in fast adaptation of both the modulation scheme and instantaneous code rate to provide a data rate suitable for current radio conditions.
Rate control is not applicable during retransmission attempts. This means that that the transport block size and the modulation scheme as well as the number of channelization codes cannot change during retransmission. Thus, in degrading channel conditions, the retransmission of a data packet that was unsuccessfully received uses the same modulation scheme and instantaneous code rate as the first transmission. In such scenario, the data packet is never successfully received, and retransmission will continue until a maximum retry count has been reached on the physical layer and eventually the data retransmission has to be performed on the higher layers such as the radio link control (RLC) level.